Supernatural immortality

Most theories of immortality are supernatural. This is because they need some supernatural concept like a soul or spirit to tie one body to the next. We can imagine many different supernatural concepts with many different random properties. The more we can imagine, the less likely any particular one is possible or true. Some people have said that because a soul like concept is supernatural you cannot really know about it. If you can not know about it how can you know that it exists beyond in peoples imagination? People have speculated about the properties of soul or spirits for a very long time and still there are no proofs of any property about them being truer than any other.

If a soul exists what is the proof that it exists long enough to be considered immortal?

Why should a soul not transform into some else's soul, equivalent to your matter in your body being part of another persons body?